The major objective of this proposal is to test the "dual T cell system" hypothesis, which proposes that the thymus functions as a compound organ to process 2 sets of thymocyte progenitors and to produce 2 lineages of T cells, one from thymus cortex and one from thymus medulla. This hypothesis derives primarily from our description of TdT plus TdT minus subsets of prothymocytes and of antigenically discrete subsets of thymocytes and T cells in rat and mouse. The ability to test this hypothesis has been enhanced by our finding that Thy-1 antigen is a selective marker for immature lymphohemopoietic cells in the rat, and by our use of the fluorescence-activated cell sorter (FACS) to isolated pluripotent hemopoietic stem cells (HSC) (75% purity) and TdT plus thymocyte progenitors (85% purity) from rat bone marrow. Our specific aims are to completely purify pluriopotent hemopoietic stem cells; to determine whether a common stem cell exists for T and B lymphocytes; to purify the TdT subset of thymocytes progenitors; to isolate the presumptive precursors of cortical and medullary thymocytes from the subcapsular region of the thymus and to purify the Thy-1 plus and Thy-1 minus subsets of rat T cells for further functional studies. The major experimental approach will be through the use of the fluorescence-activated cell sorter in concert with antisera to a variety of cell surface differentiation antigens. Additionally we plan a series of experiments on B cell ontogeny, based on observation that in the rat Thy-1 antigen is present on B cells only during the antigen independent phase of development. Using the FACS to separate immature subsets of the B cell lineage we propose to determine whether a discrete progenitor for B lymphocytes exists, and the stages in development at which B cells become readily tolerizable, and at which they first can be stimulated by antigen to generate antibody-forming cells. Our ultimate goal is to test the developmental and functional potential of these purified cell subsets using in vivo and in vitro cloning systems to gain a more comprehensive view of the organization of the lymphocyte system and the factors which control its development and function.